NASA’s Chandra X-ray Observatory has discovered part of an
intergalactic web of hot gas and dark matter that contains most of
the material in the universe. The hot gas, which appears to lie like
a fog in channels carved by rivers of gravity, has been hidden from
view since the time galaxies formed.

“The Chandra observations, together with ultraviolet
observations, are a major advance in our understanding of how the
universe evolved over the last 10 billion years,” said Fabrizio
Nicastro of the Harvard-Smithsonian Center for Astrophysics (CfA) in
Cambridge, Mass. and head of one of the teams of scientists involved
in the discovery.

Four independent teams of scientists, whose results appear as
separate papers in The Astrophysical Journal, used Chandra to detect
intergalactic gas with temperatures ranging from 300,000 to 5 million
degrees Celsius. This gas forms part of a gigantic system of hot gas
and dark matter that defines the cosmic landscape. The gaseous
component alone contains more material than all the stars in the
universe.

“We had strong suspicions from the Big Bang theory and
observations of the early universe that this gas exists in the
present era, but like a stealth aircraft it had eluded our
detection,” said Claude Canizares of the Massachusetts
Institute of Technology (MIT), who along with Taotao Fang, led of one
of the teams.

The hot gas detected by Chandra can be used to trace the presence of
the more massive dark matter component. The discovery of the hot gas
may eventually enable astronomers to map of the distribution of dark
matter in the universe and perhaps understand its origin.

Ultraviolet telescopes had detected cooler components of the hot gas
system, but because of its high temperatures most of it is detectable
only with an extremely sensitive X-ray telescope. The various groups
used two techniques to probe the intergalactic gas. One method uses
the absorbing effects of the gas on X-rays from distant
galaxies.

On their way to Earth, the X-rays from a distant quasar dim as they
pass through a cloud of the intergalactic gas. By measuring the
amount of dimming due to oxygen and other elements in the cloud,
astronomers were able to estimate the temperature, density and mass
of the absorbing gas.

Observations of the quasars PKS 2155-304 by the MIT and
Harvard-Smithsonian groups, and H1821+643 by an Ohio State group,
revealed various parts of the hot gas system. One of these appears to
be a filament in which the Milky Way and Andromeda galaxies are
embedded, whereas other detected portions are at distances of a few
billion light years from Earth.

Stills from Rivers of Gravity Animation

These results confirm earlier work by Joel Bregman and Jimmy Irwin of
the University of Michigan in Ann Arbor, who flipped the normal
procedure, and used the fact that the hot gas is itself a source of
X-rays. By observing the absorption of X-rays from the hot gas by a
foreground galaxy, they were able to deduce presence of hot gas
behind the galaxy.

“Normally the doctor studies the X-ray shadow produced by your
bones to learn about your bones,” said Bregman. “In
essence, we used the shadow to learn about the X-ray
machine.”

During the first few billion years of the universe, about 20 percent
of the matter came together under the influence of gravity to form
groups and clusters of galaxies. Theories predict that most of the
remaining normal matter and dark matter formed an immense filamentary
web connecting the groups and clusters of galaxies, predicted to be
so hot that it would be invisible to optical, infrared, and radio
telescopes.

“Computer simulations have been telling us for several years
that most of the ‘missing’ gas in the universe should be
in hot filaments,“ said Smita Mathur, leader of the Ohio State
team. “Most of those filaments are too faint to see, but it
looks like we are finally finding their shadows.”